Incandescent light bulbs are quickly being replaced by light-emitting diodes (LEDs), in particular, in automotive market. This is because LED technology provides greatly improved energy efficiency, better reliability, lowered cost and smaller form factor compared with incandescent light bulbs. LEDs are typically packaged as surface-mount device (SMD) which allows for high volume, low cost printed circuit board (PCB) manufacturing along with ever advanced semiconductor technology, further reducing production cost. LED lighting generates less heat which further reduces environment cooling requirement and cost. With LED integration into the automotive market, fuel efficiency can be improved for longer cruising range and lower fuel cost.
Many LED applications require a dimming function. One method of achieving LED dimming function is to adjust the LED forward current. However, it is well known that LED's light spectrum is also dependent on LED's forward current. Reducing the LED current to reduce LED brightness to achieve dimming function also unwantedly shifts the LED color, which is undesirable.
Therefore, LED dimming function is typically implemented using the PWM (Pulse Width Modulation) method where the nominal forward current is applied to the LED but the forward current is switched on and off periodically so that the root mean square (RMS) current value can be adjusted to the desired value. Because the forward current remains at same nominal current value, the LED color will remain the same across the full brightness controlled range. The PWM dimming frequency is usually above 100 to 120 Hz to avoid visual flickering, and a PWM dimming frequency of around 200 Hz is typically used. Although, higher frequencies can be used, high PWM switching frequency will have higher switching power loss, as well as more harmonic electromagnetic interference (EMI) emission into frequency range which may interfere with adjacent RF circuit operation.
FIG. 1 is a schematic diagram illustrating one example of an LED lighting application. Referring to FIG. 1, a string of LEDs 2 is connected to an LED controller 1. The LED string 2 is connected to a switch SW which is driven by a PWM signal, which can be either from the system control unit or from the LED controller itself. The LED controller 1 provides the forward current to the LED string 2. By turning the switch SW on and off at different duty cycle, the brightness emitted by the LED string can be controlled to achieve the dimming function. However, in the typical applications, implementing LED dimming by PWM switching sometimes leads to undesirable side effects.
In particular, the LED controller 1 receives a power supply voltage VDD. An input capacitor Cin is coupled to the power supply voltage VDD to filter out the power supply voltage. The input capacitor Cin is typically a low cost ceramic capacitor. When the dimming function is implemented, the PWM signal switches the LED forward current on and off at the same switching frequency if VDD regulation is not able to respond quickly enough. This pulsed current is seen by the ceramic input capacitor connected to the VDD power rail, causing the input capacitor to resonate mechanically due to the piezoelectric effect. With sufficiently large LED current being turned on or off, large voltage ripple can be developed on VDD power rail to cause the input capacitor to resonate at the PWM frequency, thereby generating audible noise since the PWM frequency is within the audible frequency range of human hearing.
The audible noise issue can be mitigated by using properly designed PCB layout and mechanical set up. For example, an LED lighting application can be implemented by placing two identical capacitors on both sides of PCB to cancel the piezoelectric effect. Alternately, the input capacitor mechanical resonance can be reduced by drilling holes besides the capacitor's soldering points. However, it is often not possible to implement these solutions as they require larger PCB layout and dual side surface-mount manufacturing adds component cost and production cost. In other examples, the ceramic input capacitor can be replaced by multilayer ceramic chip capacitor (MLCC) or electrolytic capacitor that does not exhibit piezoelectric behavior, thereby avoiding the audible noise altogether. However, these capacitors are more expensive than ceramic capacitors and therefore increases the component cost.
Another solution to the audible noise issue with LED dimming function involves the use of power supply voltage isolation and an output capacitor Cout coupled to the LED string, as shown in FIG. 2. FIG. 2 is a schematic diagram illustrating another example of an LED lighting application. Referring to FIG. 2, a string of LEDs 2 is connected to an LED controller 3. The LED string 2 is connected to a switch SW which is integrated into the LED controller 3 in the present example. Integrating the switch SW into the LED controller reduces component cost and also enables more precise control of the LED current, such as by use of a constant current source. The switch SW is driven by a PWM signal to switch the LED forward current on and off to achieve the dimming function. The LED controller 3 receives a power supply voltage VDD which is also coupled to an input capacitor Cin. The LED controller includes a voltage regulation circuit 4 to isolate the anode of the LEDs (node 5) from the power supply voltage VDD. An output capacitor Cout is connected in parallel to the LED string 2. Accordingly, voltage ripple at the input capacitor is eliminated by the use of the voltage regulation circuit 4 and the output capacitor Cout absorbs the power ripple across the LEDs 2. Examples of the voltage regulation circuit that can be incorporated into the LED controller include a low-dropout (LDO) voltage regulator, a charge pump, a buck regulator, or a boost regulator. Other voltage regulation circuits can also be used.
As thus configured, PWM function is achieved by turning switch SW on and off at the PWM frequency. Power ripple across the LED string 2 can be absorbed by the output capacitor Cout. When driving large LED current, the capacitance of the output capacitor Cout has to be increased proportionally, otherwise the voltage on Cout itself will generate ripple, becoming another audible noise source. In the application shown in FIG. 2, the current flowing into switch SW is identical to the current flowing into the LED string 2. When the LED current is large, the conduction loss incurred in switch SW can be large, causing system power loss. To minimize this conduction loss, the resistance of switch SW has to be minimized.
In many applications, the LED controller may be configured to drive multiple strings of LEDs. In some cases, the LED strings are powered directly by the power rail VDD and the LED controller controls the LED forward current to achieve constant current at each LED string. When multiple LED strings are used, the LED current becomes very large, which can cause large ripple on the power rail. Thus, the audible noise issue caused by the LED dimming function becomes even more serious.
Other solutions to the audible noise issue in LED dimming function include coupling a switch in series with the output capacitor, as described in U.S. Patent Publication Application No. 2012/0235596. Another solution involves shifting the PWM frequency to above the human audible range, that is, above 20 KHz, as described in U.S. Pat. No. 8,994,277. Although shifting the PWM frequency out of the human audible range can completely obviate the audible noise issue in LED dimming, this method is sometimes not desirable due to electromagnetic interference (EMI) concerns when the PWM frequency is shifted to a high frequency. Faster PWM frequency will also increase operation switching loss, reducing system power efficiency. Also, ripples on the VDD power rail still exists, which may affect other devices that are sharing the same power rail. In addition, for high contrast ratio applications, e.g., 5,000:1, the LED driver circuit may not be able to switch fast enough for such a high frequency operation.
In multiple LED string systems, it is possible to apply clock skewing to spread out the clock signal emitted power, thereby reducing the peak emitted power and reducing EMI effect. In LED applications, clock skewing refers to starting the PWM cycle for each LED channel at a different time so that the LED current will not be drawn from the power supply simultaneously by the multiple LED strings. In this manner, the power transients are spread out, thereby lowering the audible noise power. For example, clock skewing can be implemented by grouping LED strings into a set of channels with the clock signal for each channel being skewed by a certain amount of time. That is, the start time of the PWM cycle for each channel is offset from the other channels but the PWM duty cycle remains the same for all the channels. Although clock skewing can be used to alleviate the EMI concern, clock skewing has limited applications due to timing constraints. For example, clock skewing cannot be used in a multiple channel, RGB LED systems, the Red, Green and Blue LED must be operational at the same time frame without any timing skew for a proper color presentation.